The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted to be prior art against the present disclosure.
In magnetic recording, as one example, reading and writing are performed by one or more heads that move relative to the surface of a storage medium. Many magnetic disk drives, for example, include a plurality of individual disks, or “platters,” which may be two-sided—i.e., each platter can store data on each of its two sides. Therefore, such a disk drive would have at least two heads for each platter. Indeed, for each platter, there is normally at least one write head and at least one separate read head, so that such a disk drive normally has at least four heads per platter.
In a common configuration, all of the heads in a given disk drive are mounted on arms attached to a common actuator that controls the radial position of the heads (an angular, tangential or circumferential component of motion is provided by the rotation of the platters relative to the heads). This is true whether there is one or many platters, and one or multiple heads per platter.
In order to control the radial position selected by the actuator, each surface of each platter has distributed upon it positional information referred to as “servo” data. The servo data are commonly distributed in spaced-apart servo “wedges” (generally spaced equiangularly) on the platter surface. By reading the servo data as each servo wedge passes under the read head, the disk drive controller can determine the precise radial and angular position of the head and can feed back that determination to control the position of the read head or the write head, depending on the required operation. Among the servo data are sync marks, which are used to determine angular position. Separate instances of the sync mark are provided at different radial positions—i.e., for different tracks.
Data on adjacent tracks may or may not be independent of one another. As areal densities for magnetic data storage continue to increase, data tracks are being written in an overlapping or “shingled” fashion—e.g., in “shingled magnetic recording” (SMR), also known as “two-dimensional magnetic recording” (TDMR)—and reading is performed using read heads or sensors having dimensions comparable to the track width. Therefore, it is important to know whether, and by how much, a read head is deviating from its nominal position relative to a track.